The present invention relates to a fluid friction clutch, and especially, fluid friction clutches which have a reserve chamber in the primary disk. Also, the present invention relates to a fan drive means for a radiator, which includes a fluid friction clutch of the above type.
A fluid friction clutch of this type is described in U.S. Pat. No. 3,262,528 and illustrated, for example, in FIG. 7 thereof. In this known fluid friction clutch, the primary disk, which includes an internal reserve chamber, rotates in the work chamber of the clutch housing (secondary part). The internal reserve chamber is connected to the work chamber through two orifices in the primary disk. One of the two orifices serves as a leakage oil bore and the other as an overflow orifice which is controlled by a movable baffle controlled as a function of the temperature. When the baffle protrudes into the work chamber, the viscous fluid is pumped out of the work chamber into the reserve chamber. Likewise, when the baffle is retracted into the reserve chamber, the fluid is forced out of the reserve chamber via the overflow orifice into the work chamber due to centrifugal force. With reference to FIG. 7 of U.S. Pat. No. 3,262,528, a fluid stream directed counterclockwise is thus produced in the work chamber of the clutch. The fluid, therefore, flows, first of all, radially outward of the overflow orifice, then passes from the rear part of the work chamber via the cylindrical circumferential part of the primary disk into the front part of the work chamber, that is to say, onto the front end face of the primary disk. There the fluid is conveyed radially inward, counter to the centrifugal force, by conveying devices (pump grooves), until it enters the reserve chamber again via the radially internal edge of the primary disk. There is superimposed upon this stream of fluid mentioned, controlled by the movable baffle, a second stream of fluid due to the leakage oil bore in the primary disk. This permanent leakage oil stream is provided for cooling purposes, in order to carry away the resulting heat in the case of excessive internal temperature, due to excessive slip, for example. One disadvantage of this known clutch is the relatively high no-load speed, which results, on the one hand, from the permanent leakage stream or cooling stream, and, on the other hand, from the baffle being located relatively far inward radially, and thereby, not being capable of pumping all the fluid out of the work chamber into the reserve chamber. Furthermore, the relatively high no-load speed of this clutch also means an unnecessary power consumption, which is undesirable. Lastly, due to its friction against the work surface on the secondary side, the baffle constitutes a wearing part with a limited useful life.